In recent years, portable electronic appliances such as a notebook-sized personal computer, a cellular phone and a personal digital assistance have spread wide. Recently prolongation of service time of portable electronic appliances and shortening of charging time (i.e., improvement of rate characteristics) thereof are eagerly desired. To fulfill these desires, requirements for rendering high in performance of battery, especially enhancing the capacity and the rate of charge, are becoming severe.
A lithium ion secondary battery has a structure such that a positive electrode and a negative electrode with a separator interposed between the electrodes are placed together with an electrolyte liquid in a vessel. The positive electrode and the negative electrode are made by bonding an electrode active material (hereinafter referred to merely as “active material” when appropriate) and an optional electrical conductivity-imparting agent and other ingredients to a collector made of, for example, aluminum or copper through a binder for electrode (hereinafter referred to as “binder” when appropriate). The bonding of an electrode material to the collector is conducted by a procedure wherein an active material and other optional ingredients are mixed with a solution or dispersion of a binder in a liquid medium to prepare a slurry composition for an electrode of lithium ion secondary battery; a collector is coated with the slurry composition; and the liquid medium is removed from the thus-formed liquid coating, for example, by drying, to form a mixed material layer comprising the active material on the collector.
The capacity of a battery greatly depends upon the amount of an active material filled in the electrode. The rate characteristics of a battery vary depending upon the ease in movement of electrons, and the rate characteristics can be enhanced by an increase of the amount of an electrical conductivity-imparting agent such as carbon. To increase the amount of an active material and the amount of an electrical conductivity-imparting agent within a limited space of battery, the amount of a binder must be minimized. However, minimization of the amount of binder results in reduction of bonding force of the active material. Therefore, a binder exhibiting an enhanced bonding force even when it is used in a minor amount is eagerly desired.
Heretofore, a fluorine-containing polymer such as polyvinylidene fluoride has been widely used as a binder for a positive electrode of a lithium ion secondary battery. However, the fluorine-containing polymer does not have a sufficiently high bonding force and flexibility, and therefore, enhancement in the capacity of battery and the rate characteristics thereof is difficult to attain.
To remedy the drawbacks of a fluorine-containing polymer, a polymer rubber has been proposed as a binder in Japanese Unexamined Patent Publication No. H4-255670. A polymer rubber exhibits good bonding force and flexibility when an electrode is made using the polymer rubber, but, the cycle characteristics of battery are poor, and the capacity of battery is reduced and the rate characteristics are deteriorated, at repetition of a charge-discharge cycle. This would be due to the fact that the binder is swollen with an electrolyte liquid, and consequently, the bonding force of binder is gradually reduced and an active material tends to be separated from a collector, and the binder undesirably covers the entire surface of collector leading to reduction in a movement of electrons.
Thus, it has been difficult to enhance both of the capacity of battery and the rate characteristics thereof.